Hyper-environmental radome and the like



Dec. 20, 1966 o, CALDWELL ETAL. 3,292,544

HYPER-ENVIRONMENTAL RADOME AND THE LIKE Filed May 5, 1964 M w 0 E E L LW m w Z L @//H/////// ///.H/ V

firmeA/Em.

United States Patent 0 f 3,292,544 HYPER-ENVIRGNMENTAL RADOME AND THELIKE Orval G. Caldwell, Los Angeles, and Leon J. Le Clercq,

Glendale, Calif., assiguors, by mesne assignments, to Douglas AircraftCompany, Inc, Santa Monica, Calif., a corporation of Deiaware Filed May5, 1964, Ser- No. 364,949 16 Claims. (Cl. 10Z-92.5)

This invention pertains to rigid unitary nose cones, rad-omes and otherformed objects through which high frequency energy must be transmitted,the objects of the present invention having highly desirable electricalcharacteristics, high strength and ability to withstand thermal shockwithout being excessively heavy. The invention also pertains to methodswhereby the unitary objects herein disclosed can be effectively andeconomically manufactured, as well as data concerning compositions,ingredients, materials and techniques of manufacture.

Radomes have been previously made of resinous and plastic materials, butsuch radomes are unsuited for use at supersonic speeds or on missilesand space vehicles, primarily because the materials are incapable offulfilling the mechanical, thermal and aerodynamic and erosionconsiderations while maintaining their electrical characteristics.Attempts have been made to utilize refractory compositions which canwithstand rap-id and widely ranging temperature changes, but thesematerials are heavy (have a high specific gravity) and in order toprovide a radome having suflicient strength to withstand maximumanticipated loads without the use of reinforcing ribs, disconformities,the use of binders, al kalies and other materials which affect otherproperties such as electrical characteristics, a radome would have to bemade with walls that are thick and diamond ground after firing toprecise $0.001 in dimensions. The radome is difficult and expensive tomake and will now add an excessive dead load to the missile or vehicle.A heavy radome, when used on a rocket, missile or other space vehiclerequires a great increase in the thrust generating, propelling facilityof the missile or vehicle, it being estimated that an added ten poundload requires an added 100 pounds of fuel. Dense radomes of refractoryoxides, even of one-half wave thickness, were either excessively heavyor incapable of withstanding hyper-environmental a plications, and atall events had to be made to conform to critical thickness toleranceswhich rendered their manufacture uneconomical and their use restrictedto a precisely maintained frequency.

By following the teachings of this invention, it is now possible toproduce, from highly refractory ceramic compositions, aerodynamicallysound, streamlined, relatively light-weight radomes, nose cones andother objects which have electrical characteristics of exceptionalutility. Their power transmission coefficients are high and lossesattributable to reflection and absorption are reduced to a minimum: theyexhibit broadband electrical performance. These and othercharacteristics such as strength, resistance to thermal shock, stabilityunder adverse conditions of use and ease of manufacture industriallyrender the products of this invention particularly adapted for use invehicles and equipment operating at high speeds, in rapidly varyingmedia under changing load conditions and/ or under rapidly changingtemperature conditions.

Generally stated the final product of this invention be it a radome,nose cone, scanner housing or other object, is composed essentially ofrefractory inorganic oxides or mixtures of oxides which are virtuallyinert and the final product is able to maintain its shape and strengthat very high temperatures (1400-1600 0.). Although the chemicalcomposition is homogeneous Patented Dec. 20, 1966 "ice throughout thebody of the product, its apparent density is varied in a controlled andpredetermined manner from one specific zone to another, therebydeveloping adjacent zones of different, predetermined physical andelectrical properties in such product.

For example, the final product can have thin surface layers of highdensity, high dielectric coefficient and low loss tangent and anintermediate layer of predetermined lower dielectric coefficient andlower density, without the presence of boundary layers, adhesives orcements which might tend to impair, distort or otherwise detract fromdesired electrical and physical characteristics. The entireformed'object, even though it includes 'layers or wall portionsdiffering from each other in electrical characteristics, is ofsubstantially uniform inorganic chemical composition. By properlyproportioning the thicknesses of the intimately united layers of theunitary product, the thicknesses bearing a predetermined and controlledrela tionship to the wave length or frequency of the signal or energywhich is to be transmitted through the final object or product ashereinafter disclosed, undesired reflection and absorption is minimizedand the range and efficiency of the equipment associated with theradomehoused unit is maximized. Such radome is of high strength andresistant to thermal shock; it is capable of being manufacturedeconomically since total wall thickness is not critical as the radomeexhibits broad band electrical performance.

An object of the present invention therefore is to provide formedobjects adapted to effectively transmit high frequency energy, suchobjects being relatively light in weight, of high mechanical strengthand the ability to withstand thermal shock as well as possessing highlyefficient electrical characteristics.

A further object of the invention is to disclose and providecompositions and methods whereby o give or streamlined symmetricalradomes, windows, nose cones and the like may be made of highlyrefractory ceramic ingredients and compositions without burdensomeweight.

Another object is to provide housings, electromagnetic windows andradomes which are of laminated character, but of uniform inorganicchemical composition and of homogeneous structure electrically, suchhousings and radomes being exceptionally well adapted forhyper-environmental applications.

A still further object is to disclose proportions, ratios, materials andcharacteristics which permit the production of housings and radomeshaving optimum electrical performance coupled with resistance to thermalshock.

Other objects of the invention as well as the attendant advantages willbecome apparent from the following detailed description of exemplarymethods of manufacture, compositions and forms of the invention. Inorder to facilitate description, reference will be had to the appendeddrawings in which:

FIG. 1 is a side elevation partly broken away of a typical nose cone orradome housing embodying the present invention;

FIG. 2 is an enlarged section through the wall of a radome such as isillustrated in FIG. 1;

FIGS. 3 and 4 are somewhat diagrammatic views of a plaster mold in whichthe object of the present invention can be manufactured, at differentstages of such manufacture.

FIGS. 5 and 6 are diagrammatic representations of a somewhat modifiedform of mold and die which may be used in carrying out the method of theinvention.

A typical nose cone or radome made in accordance with this invention isillustrated in FIG. 1, and is provided with a wall 10 of generallyuniform thickness having a smooth, generally convex outer surface 11 anda concave inner surface 14. As better illustrated in the enlargedpartial section of FIG. 2, the wall includes a thin outer layer 12 and athin inner layer 13. These innerand outer layers are of unitary widthand intimately bonded to a central thicker core 15. This Wall (andtherefore all layers and the entire object) is of virtually homogeneouschemical composition and more than 90% thereof (preferably 91% to 97%)is composed of an inorganic refractory such as alumina, mullite orcalcined sillimanite. The remaining solids contents totalling less than10% includes bonding minerals such as clays, crystal growth inhibitorsand mineralizers.

The particles of refractory (such as alumina) used in making the wallare finely milled to a grain size of 60 microns and smaller. In theouter and inner layers 12 and 13 these particles of alumina are inclosely adjacent relation and such layers are dense, weighing betweenabout 200 and 235 pounds per cubic foot. In the central core theparticles of alumina are arranged to form uniformly microporousstructure which may weigh 45 to 120 pounds per cubic foot. At allevents, the dense outer and inner surface layers have a dielectricconstant of 8 to 12, whereas the central core has a dielectric constantof less than 8, preferably 2 to 5 or 2.5 to 4.

Although materials having a dielectric constant of 8 or more normallyhave a low loss tangent and introduce reflection losses, theconstruction here described utilizes the excellent strength, erosionresistance, and thermal shock resistance of dense alumina and eliminatesreflection losses by making these outer and inner layers 12 and 13 sothin that they are virtually invisible to the energy Waves beingtransmitted. Pursuant to this invention, the thickness of each of layers12 and 13 is only 0.06 to about 0.04 of the full wave length of thefrequency which the housing or radome is to transmit or receive. Thecore 15, on the other hand, has a low dielectric contsant and a lowrefiec tion coeificient and the thickness of such core is not critical,but preferably is on the order of 0.3 to 0.7 of the full wave length ofthe energy to be transmitted through the wall.

The term full wave length, as used herein, is the actual wave length ofthe energy to be received or transmitted (in centimeters or inches)divided by the dielectric constant of the wall or densest component ofsuch wall.

An electromagnetic window made as herein described has numerousadvantages. It is to lighter in weight, has a very high strength toweight ratio, is resistant to erosion and thermal shock (the porous coreacts as a heat sink), is homogeneous chemically and electrically,exhibits very low absorption of energy and therefore has a highelectromagnetic transmission, and exhibits improved broad bandperformance; it is effective even when the frequency of the energytransmitted varies 15%. Moreover, the objects can be reproduciblymanufactured in a simple and efiicient manner, without resort todelicate and accurate finishing. A consideration and appreciation of theobjectives and definitions of this invention permits ready applicationof electromagnetic theory and geometrical optics to the design ofradomes having optimum electrical performance, tremendous stability andadaptability for utilization in extreme environmental conditions.Attention is again called to the fact that the overall thickness of thewall (indicated at T in FIG. 2) is preferably between 0.3 and about 0.7of the full wave length of the energy to be transmitted; the thicknessof the central core is preferably between about 0.4 and 0.6 of the fullwave length. Differently stated, the core thickness (T is between about70% and 90% of T and is not critical.

A preferred method of making the objects of this invention involvescasting the layer sequentially in porous, absorbent molds. A female moldmade from gypsum (such as a mold used for casting cups and dinnerwarebowls from clay-containing slips) may be used; such mold should be driedto a free moisture content of say 1%3% before use, and at such time willhave the capacity to absorb 18%25% of water by weight. FIG. 3illustrates an exemplary mold 20 having an inner surface 21 which shouldbe smooth, free from dust and of a contour corresponding to the outercontour of the finished radome or other object, allowance being made forshrinkage which may take place when the cast object is fired.

A casting slip or suspension is prepared containing, as inorganicrefractory oxides and solids, from to about 97% by weight of finelymilled alumina (although mullitc or calcined sillimanite or otherrefractory oxides could be used) and from about 3% to 10% by weight ofbinding minerals such as clays and mineralizers and crystal growthinhibitors. The alumina should have a grain size not exceeding 60microns; classified mixtures can be used. Binding clays preferablyinclude those from the montmorillonite group, particularly of thetrioetahedral type, such as hectorite from which substantially allnormal CaCO contaimination has been removed, and ball type clays. Minorquantities of magnesium and calcium silicate minerals and mineralizersand crystalgrowth inhibitors such as BaSiF can be used. Typical solidscompositions are s The finely ground olay type minerals are preferablyfirst suspended in water and the alumina and mineralizers then added andintimately mixed to form a smooth slurry or suspension. Water content isadjusted to produce a slip-of desired viscosity and specific gravity(between about 2.0 and 2.2). The slip may be deareated or may containminute quantities of modifying agents such as polyvinyl alcohol or gumsto control rheological properties. The slip is then SlOWllY poured intothe mold (which may be axially rotated during the casting) to form askin or layer of desired thinness on the inner surface of the mold. Thislayer, corresponding to the outer surface layer 12 of FIGS. 1 and 2, ispermitted to partially dry so that surface moisture is not visible andthe inner core is then rapidly cast directly upon the layer 12.

The suspension used in casting the core is preferably identical ininorganic SOllldS content to that used for the outer layer 12. Inaddition, such suspension new contains a predetermined proportion oforganic, preferably hollow particles of volatilizable, low ash materialsuch as synthetic resin (for example, urea-formaldehyde 0rphenol-formaldehyde spheres), such particles having imperforate wallsand being of virtually uniform selected size. Preferably these hollowballs are smaller than the refractory grain; hollow spheres of 20 and 25micron size have been used successfully. These spheres may be prewettedbefore being added to the suspension of the inorganic refractorymaterials. The utilization of such hollow spheres in producing porousrefractory bodies is fully disclosed in a copending application filed byOrval G. Caldwell, Ser. No. 182,921. By the use of these hollow plasticspheres of predetermined and substantially uniform size, it is possibleto accurately control the porosity and therefore the density of theporous structure desired for the core. Ordinarily, from about 6% to 25%by weight of the solid components of the slip constitutes the quantityof such h'OlilOW spheres added to the slip. In some instances it isdesirable to lightly spray the plastic spheres with a minute proportionof glycerin or ethylene glycol before adding them to the slip.

After the core has been cast to a desired thickness (which, aspreviously indicated, is not critical), the partially finished castingis again permitted to dry partially and the inner dense layer 13 isslowly cast, this dense layer being cast with a suspension identical tothat utilized in castin the outer layer 12. T hereafter, the casting isallowed to dry until it releases from the mold, removed, dried and thenfired to a temperature of between about 2600 F. and 2900 F., suchtemperatures being generally reached in about 18 hours. The radome orother object is now in finished form, although in some instances it isdesirable to true up and grind the base of the cone to insure a tightfitting with its mount.

For purpose of information, it may be noted that the dense layerobtained by utilizing a suspension or slip having the inorganiccomposition given in the first column of the table hereina bove whencast at a slip specific gravity of 2.15 produced an extremely strongfired layer having a dielectric constant of 12.2. Dielectric constantsof 9 and higher are readily obtained for the dense, thin layers; thespherical, plastic particles burn out without leaving residues duringthe firing and produce an electrically homogeneous, uniformly porouscore whose density may be very accurately controlled. Such layers mayhave dielectric constants of 2.5 to 4.5, depending upon their apparentdensity. A typical electromagnetic window may have dense inner and outerlayers measuring 0.025

inch in thickness and an intervening, intimately bonded,

porous core appnoximateily 0.250 inch in thickness, such electromagneticwindow being particularly well adapted for use with 3 cm. wave lengths.

It may be noted that combined drying and firing shrinkages on the orderfrom about 1.3% to about 10%12% may be expected, depending upon thedensity of the finished object and the firing temperatures employed.

Separate male and female molds may also be employed in making theobjects of the present invention. FiG- URE 5 illustrates a female mold29' having a suitably contoured surface 21 adapted to impart the desiredconfiguration to the nose cone or other object. Whereas FIGURE 6diagrammatically represents a male absorptive mold 34 having a suitablycontoured surface arranged to pnovide the interior configuration of thenose cone. This male mold is shown provided with a head 35 which extendsoutwardly, this head being adapted to rest upon the upwardly facingshoulders of the mold 20' so as to position the male mold 34 in spacedrelation with the internal surface 21 of the female mold. Locating pinsmay be employed (not shown) to insure proper positioning of the malemold wit-h respect to the female lTlOtld.

After the two mold portions 20 and 34 are placed in position, oneextending into the other, the thin inner and outer dense surface layersof the refractory composition may be cast on the internal surface 21 ofthe female mold and the external surface of the male mold 34. This canbe readily accomplished by providing one or more supply lines 36 throughwhich this suspension may be rapidly fed into the cavity between the twomold surfaces. After a predetermined length of time the molds are againseparated and the slip or suspension poured off, thereby leaving a layerof desired thickness on each of the two surfaces. The centralmicroporous core can then be cast in a similar manner thereby unitingthe surface layers. The male mold 34 may also include an air supply line37 terminating insuitable perforated pipe sections within the male moldso that after the entire object has been cast compressed air may besupplied through line 37 in order to facilitate the separation of themold from the cast object.

When very large radomes, cones or other objects are. being made the mold20 may be used for forming the thin outer layer of the object and themale absorptive mold 34 may be dipped into a suspension or slip of thesame composition so as to separately form on such male mold the thindense layer of refractory. Thereafter, the now coated male mold 34 maybe placed in position upon the female mold 20' (whose inner surface hasalready been coated with a dense layer) and then the core of microporousrefractory composition may be cast in position to form a unitary object.

In some instances greater homogeneity is assured by casting the thindense outer layer upon the surface of a female mold then placing anonporous male mandrel or die within the female mold and casting themicroporous low density core into the space between the mandrel andcoated female mold. After this core is cast the mandrel or die isremoved and the inner dense .layer is applied to the inwardly facingsurface of the microporous layer. From these various alternative methodsof casting it will be evi cut that many objects can be readily made bysuccessive casting or refractory compositions difiering in content ofhollow resinous particles, the number of layers and the respectivethickness and densities being under accurate control. It is to be notedthat the adjacent layers are parallel to a major surface of the objectdifferent in density and electrical characteristics and are intimatelybonded to each other.

Radomes and other electromagnetic windows designed and manufactured inthe manner herein described have capabilities adapting them for use tohyper-environmental conditions. They are capable of being used forcommunication, navigation, guidance, identification, tracking,telemetry, radar, comimand-destruct systems and in many otherapplications wherein the windows are subjected to thermal shock, erosionand other adverse conditions resulting from supersonic speeds andadverse environments. Although a particular laminated arrangement hasbeen disclosed, the presence of an additional very thin and dense layermedially of the porous core layer, and multilayered objects of otherarrangements can be made by the methods here described and are withinthe contemplation of this invention.

We claim:

1. A strong, unitary radome, nose cone and similar formed object adaptedto effectively transmit high frequency energy and hawin g the ability towithstand thermal shock comprising: a rigid contoured object including awall having a generally convex outer surface and a generally concaveinner surface, said wall being composed essentially and virtuallyhomogeneously of particles of refractory oxide bonded together by asmall quantity of fritted minerals including clay of the montmorillonitegroup; the particles of refractory oxide being in compact, closelyadjacent relation at said inner and outer surfaces to form thin denselayers of predetermined and virtually uniform thickness at saidsurfaces, said surface layers having a dielectric constant of 9 andhigher and a low loss tangent; the particles of refractory oxide in thewall portion between said surface layers being arranged and bondedtogether to form a uniformly porous structure of low apparent densityhaving a predetermined dielectric constant of less than 9; the overallthickness of said wall being between about 0.3 and 0.7 of the full Wavelength of the energy to be transmit-ted through the wall of the object,said low density wall portion constituting between 70% and of saidoverall wall thickness.

2. A unitary object as stated in claim 1, wherein each of the denselayers has a thickness not exceeding about 0.06 of the full wave lengthof the energy to 'be transmitted.

3. A unitary radome as stated in claim 1, wherein the wall is composedof more than 90 percent alumina and less than 10 percent of mineralsincluding hectorite clay.

4. A unitary radome as stated in claim 1, wherein the wall of the radomeis composed of more than 93 percent of a refractory particle having agrain size smaller than 60 microns and less than 6 percent of bondingminerals and crystal growth inhibitors.

5. A strong, lightweight, unitary 'radome, nose cone, electromagneticwindow and similar formed object adapted to effectively transmit highfrequency energy and having the ability to withstand thermal shockcomprising: a rigid contoured object including a wall having a generallyconvex outer surface and a generally concave inner surface, said wallbeing of virtually uniform overall thickness; said wall being com-posedessentially and virtually homogeneously of particles of refractory oxidebonded together by a small quantity of fired products of mineralsincluding clay; the particles of refractory oxide being in compact,closely adjacent relation at said inner and outer surfaces to form thindense layers having a dielectric constant of 9 and higher and a low losstangent; the particles of refractory oxide in the wall portion betweensaid surface layers being arranged and bonded together to form auniformly porous structure of low apparent density having apredetermined dielectric constant of between about 2 and 5, thethickness of said low density centrally disposed wall portion beingbetween about 0.4 and 0.6 of the full wave length of the energy to betransmitted through the wall of the object; the overall thickness of thewall being between about 0.3 and 0.7 of said full wave length. r

6. An electromagnetic window as stated in claim 5, wherein said wall ischemically homogeneous, is free from organic components and containsmore than 90% alumina, and each of said thin dense layers has athickness not exceeding about :06 of the full wave length of the energyto be transmitted.

7. A method of making unitary, refractory objects composed essentiallyof particles of refractory oxides bonded by small quantities of frit-tedminerals including clay, said object being of virtually uniform chemicalcomposition but having continuous adjacent layers, differing in densityand integrally bonded to each other, comprising: making a first aqueoussuspension composed of finely divided refractory oxides in majorproportion and a minor proportion of bonding minerals; making a secondaqueous suspension composed of timely divided refractory oxides andbonding minerals in substantially the same proportions as the firstsuspension, but containing a predetermined added amount of hollowsynthetic resin composition particles; sequentially casting said aqueoussuspensions to form adjoinin contacting layers of desired thickness, andthen firing the cast object to bond said layers together.

8. A strong, unitary radome, nose cone and similar formed object adaptedto effectively transmit high frequency energy and having the ability towithstand thermal shock comprising: a rigid contoured object including awall having a generally convex outer surface and a generally concaveinner surface, said wall being composed essentially and virtuallyhomogeneously of particles of refractory oxide bonded together by asmall quantity of fired products of minerals including clay; theparticles of refractory oxide being in compact, closely adjacentrelation at said inner and outer surfaces to form thin dense layers ofpredetermined and virtually uniform thickness at said surfaces, theparticles of refractory oxide in the Wall portion between said surfacelayers being arranged and bonded together to form a uniformly porousstructure of lower density than said surface layers, said surface layershaving a substantially higher dielectric constant than said wallportion, said wall portion constituting between 70% and 90% of saidoverall wall thickness.

9. A unitary object as defined in claim 8, wherein each of the densesurface layers has a dielectric constant of 8 to 12, and said wallportion between said layers has a dielectric constant of less than 8.

.10. A unitary object as defined in claim 9, wherein said wall portionhas a dielectric constant of 2 to 5.

11. A unitary object as defined in claim 8, wherein the overallthickness of said wall is between about 0.3 and 8 0.7 of the full wavelength of the energy to be transmitted through the wall of said object,and each of said thin dense surface layers has a thickness not exceedingabout 0.06 of said full Wave length.

12. A method as defined in claim 7, wherein said first and secondaqueous suspensions each contain from to about 97% by weight of saidrefractory oxides and from about 3% to 10% by weight of said bondingminerals, and wherein said second aqueous suspension also contains fromabout 16% to 25% of said hollow synthetic resin composition particles,by weight of the solid components of said second suspension.

13. A method as defined in claim 7, wherein said refractory oxides areselected from the group consisting of alumina, mullite and calcinedsillirnanite, and said bonding minerals include a clay from themontmorillonite group.

14-. A method as defined in claim 7, wherein said refractory oxides arealumina having a grain size not in excess of 60 microns, said bondingminerals include hectorite, and said hollow synthetic resin compositionparticles are selected from the group consisting of urea formaldehydeand phenol-formaldehyde spheres.

15. A method of making unitary, refractory objects composed essentiallyof particles of refractory oxides bonded by small quantities of frittedminerals including clay, said object being of virtually uniform chemicalcomposition but having continuous adjacent layers, differing in densityand integrally bonded to each other, comprising: making a first aqueoussuspension composed of finely divided refractory oxides in majorproportion and a minor proportion of bonding minerals; casting saidfirst aqueous suspension on a surface to form a first :thin layer,making a second aqueous suspension composed of finely divided refractoryoxides and bonding minerals in substantially the same proportions as thefirst suspension, but containing a predetermined added amount of hollowsynthetic resin composition particles; casting said second aqueoussuspension upon said first layer to form a core of substantially greaterthickness than said first layer, and casting said first aqueoussuspension on said core to form a second thin layer, and then firing thecast object to bond said layers and core together.

16. A unitary, strong, refractory object composed essentially ofparticles of refractory oxide bonded by small quantities of mineralsincluding clay, said object being of substantially uniform chemicalcomposition throughout but composed of continuous, adjacent layersincluding thin dense inner and outer layers and an intermediate thickercore layer of porous structure having substantially lower density thansaid thin inner and outer layers, said inner and outer layers having ahigher dielectric constant than said intermediate core layer, saidlayers being intimately bonded together.

References Cited by the Examiner UNITED STATES PATENTS 2,962,717 11/1960Kofoid 343872 BENJAMIN A. BORCHELT, Primary Examiner.

FRED C. MATTERN, 112., Examiner.

R. F. STAHL, Assistant Examiner.

1. A STRONG, UNITARY RADOME, NOSE CONE AND SIMILAR FORMED OBJECT ADAPTEDTO EFFECTIVELY TRANSMIT HIGH FREQUENCY ENERGY AND HAVING THE ABILITY TOWITHSTAND THERMAL SHOCK COMPRISING: A RIGID CONTOURED OBJECT INCLUDING AWALL HAVING A GENERALLY CONVEX OUTER SURFACE AND A GENERALLY CONCAVEINNER SURFACE, SAID WALL BEING COMPOSED ESSENTIALLY AND VIRTUALLYHOMOGENEOUSLY OF PARTICLES OF REFRACTORY OXIDE BONDED TOGETHER BY ASMALL QUANTITY OF FRITTED MINERALS INCLUDING CLAY OF THE MONTMORILLONITEGROUP; THE PARTICLES OF REFRACTORY OXIDE BEING IN COMPACT, CLOSELYADJACENT RELATION AT SAID INNER AND OUTER SURFACES TO FORM THIN DENSELAYERS OF PREDETERMINED AND VIRTUALLY UNIFORM THICKNESS AT SAIDSURFACES, SAID SURFACE LAYERS HAVING A DIELECTRIC CONSTANT OF 9 ANDHIGHER AND A LOW LOSS TANGENT; THE PARTICLES OF REFRACTORY OXIDE IN THEWALL PORTION BETWEEN SAID SURFACE LAYERS BEING ARRANGED AND BONDEDTOGETHER TO FORM A UNIFORMLY POROUS STRUCTURE OF LOW APPARENT DENSITYHAVING A PREDETERMINED DIELECTRIC CONSTANT OF LESS THAN 9; THE OVERALLYTHICKNESS OF SAID WALL BEING BETWEEN ABOUT 0.3 AND 0.7 OF THE FULL WAVELENGTH OF THE ENERGY TO BE TRANSMITTED THROUGH THE WALL OF THE OBJECT,SAID LOW DENSITY WALL PORTION CONSTITUTING BETWEEN 70% AND 90% OF SAIDOVERALL WALL THICKNESS.